ipfs/paper/gfs.tex

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\begin{document}

% \conferenceinfo{WOODSTOCK}{'97 El Paso, Texas USA}

\title{Galactic File System}
\subtitle{}

\numberofauthors{1}

\author{
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  Juan Benet\\
  \email{juan@benet.ai}
}

\maketitle
\begin{abstract}
The Galactic File System is a peer-to-peer distributed file system capable of
sharing the same files with millions of nodes. GFS combines a distributed
hashtable, cryptographic techniques, merkle trees, content-addressable
storage, bittorrent, and tag-based filesystems to build a single massive
file system shared between peers. GFS has no single point of failure, and
nodes do not need to trust each other.
\end{abstract}

\section{Introduction}

Cite:
CFS
Kademlia
Bittorrent
Chord
DHash
SFS
Ori

\section{Design}

\subsection{GFS Nodes}

GFS is a distributed file system where all nodes are the same. They are
identified by a \texttt{NodeId}, the cryptographic hash of a public-key
(note that \textit{checksum} will henceforth refer specifically to crypographic
hashes of an object). Nodes also store their public + private keys. Clients are
free to instatiate a new node on every launch, though that means losing any
accrued benefits. It is recommended that nodes remain the same.

\begin{verbatim}
      type Node struct {
        id NodeID
        pubkey PublicKey
        prikey PrivateKey
      }
\end{verbatim}


Together, the
nodes store the GFS files in local storage, and send files to each other.
GFS implements its features by combining several subsystems with many
desirable properties:

\begin{enumerate}
  \item A Coral-based \textbf{Distributed Sloppy Hash Table}\\
        (DSHT) to link and coordinate peer-to-peer nodes.
  \item A Bittorrent-like peer-to-peer \textbf{Block Exchange} (BE) distribute
        Blocks efficiently, and to incentivize replication.
  \item A Git-inspired \textbf{Object Model} (OM) to represent the filesystem.
  \item An SFS-based self-certifying name system.
\end{enumerate}


These subsystems are not independent. They are well integrated and leverage
their blended properties. However, it is useful to describe them separately,
building the system from the bottom up. Note that all GFS nodes are identical,
and run the same program.

\subsection{Distributed Sloppy Hash Table}

First, GFS nodes implement a DSHT based on Kademlia and Coral to coordinate
and identify which nodes can serve a particular block of data.

\subsubsection{Kademlia DHT}

Kademlia is a DHT that provides:

\begin{enumerate}

  \item Efficient lookup through massive networks:
        queries on average contact $ \ceil{log_2 (n)} $ nodes.
        (e.g. $20$ hops for a network of $10000000$ nodes).

  \item Low coordination overhead: it optimizes the number of
        control messages it sends to other nodes.

  \item Resistance to various attacks, by preferring nodes who have been
        part of the DHT longer.

  \item wide useage in peer-to-peer applications, including Gnutella and
        Bittorrent, forming networks of over 100 million nodes.

 \end{enumerate}

While some peer-to-peer filesystems store data blocks directly in DHTs,
this ``wastes storage and bandwidth, as data must be stored at nodes where it
is not needed''. Instead, GFS stores a list of peers that can provide the data block.

\subsubsection{Coral DSHT}

Coral extends Kademlia in three particularly important ways:

\begin{enumerate}

  \item Kademlia stores values in nodes whose ids are ``nearest'' (using
        XOR-distance) to the key. This does not take into account application
        data locality, ignores ``far'' nodes who may already have the data, and
        forces ``nearest'' nodes to store it, whether they need it or not.
        This wastes significant storage and bandwith. Instead, Coral stores
        addresses to peers who can provide the data blocks.

  \item Coral relaxes the DHT API from \texttt{get\_value(key)} to
        \texttt{get\_any\_values(key)} (the ``sloppy'' in DSHT).
        This still works since Coral users only need a single (working) peer,
        not the complete list. In return, Coral can distribute only subsets of
        the values to the ``nearest'' nodes, avoiding hot-spots (overloading
        \textit{all the nearest nodes} when a key becomes popular).

  \item Additionally, Coral organizes a hierarchy of separate DSHTs called
        \textit{clusters} depending on region and size. This enables nodes to
        query peers in their region first, ``finding nearby data without
        querying distant nodes'' and greatly reducing the latency of
        lookups.

\end{enumerate}


\subsubsection{GFS DSHT}

The GFS DSHT supports four RPC calls:



\subsection{Block Exchange - BitSwap Protocol}

The exchange of data in GFS happens by exchanging blocks with peers using a
BitTorrent inspired protocol: BitSwap. Like BitTorrent, BitSwap peers are
looking to acquire a set of blocks, and have blocks to offer in exchange.
Unlike BitTorrent, BitSwap is not limited to the blocks in one torrent.
BitSwap operates as a persistent marketplace where node can acquire the
blocks they need, regardless of what files the blocks are part of. The
blocks could come from completely unrelated files in the filesystem.
But nodes come together to barter in the marketplace.

While the notion of a barter system implies a virtual currency could be
created, this would require a global ledger (blockchain) to track ownership
and transfer of the currency. This will be explored in a future paper.

Instead, BitSwap nodes have to provide direct value to each other
in the form of blocks. This works fine when the distribution of blocks across
nodes is such that they have the complements, what each other wants. This will
seldom be the case. Instead, it is more likely that nodes must \textit{work}
for their blocks. In the case that a node has nothing that its peers want (or
nothing at all), it seeks the pieces its peers might want, with lower
priority. This incentivizes nodes to cache and disseminate rare pieces, even
if they are not interested in them directly.

\subsubsection{BitSwap Credit}

The protocol must also incentivize nodes to seed when they do not need
anything in particular, as they might have the blocks others want. Thus,
BitFlow nodes send blocks to their peers, optimistically expecting the debt to
be repaid. But, leeches (free-loading nodes that never share) must be avoided. A simple credit-like system solves the problem:

\begin{enumerate}
  \item Peers track their balance (in bytes verified) with other nodes.
  \item Peers send blocks to each other probabilistically, according to
        a function, that falls when owed and rises when owing.
  \item The sigmoid (scaled by a comparison of the ownership) provides such a
        function:

  \[ P(send) = \dfrac{1}{1 + exp(-r)} \]
  where the \textit{debt ratio} $ r $ is
  \[ r = \dfrac{\texttt{bytes\_recv} - \texttt{bytes\_sent}}{\texttt{bytes\_sent}} \]
\end{enumerate}

\begin{center}
\begin{tabular}{ >{$}c<{$} >{$}c<{$}}
  P_{send}(\;\;\;r) =& likelihood \\
  \hline
  \hline
  P_{send}(-5) =& 0.01 \\
  P_{send}(-4) =& 0.02 \\
  P_{send}(-3) =& 0.05 \\
  P_{send}(-2) =& 0.12 \\
  P_{send}(-1) =& 0.27 \\
  P_{send}(\;\;\;0) =& 0.50 \\
  P_{send}(\;\;\;1) =& 0.73 \\
  P_{send}(\;\;\;2) =& 0.88 \\
  P_{send}(\;\;\;3) =& 0.95 \\
  P_{send}(\;\;\;4) =& 0.98 \\
\end{tabular}
\end{center}

As you can see in Table 1, this function drops off quickly as the nodes' \
\textit{debt ratio} surpasses twice the established credit.
This \textit{debt ratio} is a measure of trust:
lenient to debts between nodes that have previously exchanged lots of data
successfully, and merciless to unknown, untrusted nodes. This
(a) provides resistane to attackers who would create lots of new nodes,
(b) protects previously successful trade relationships, even if one of the
nodes is temporarily unable to provide value, and
(c) eventually chokes relationships that have deteriorated until they
improve.

\subsubsection{BitSwap Ledger}

BitSwap nodes keep ledgers accounting the transfers with other nodes.
A ledger snapshot contains a pointer to the previous snapshot (its checksum),
forming a hash-chain. This allows nodes to keep track of history, and to avoid
tampering. At initializing, BitSwap nodes exchange their ledger information.
If it does not match exactly, the ledger is reinitialized from scratch,
loosing the accrued credit or debt.  It is possible for malicious nodes to
purposefully ``loose'' the Ledger, hoping the erase debts. It is unlikely that
nodes will have accrued enough debt to warrant also losing the accrued trust,
however the partner node is free to count it as \textit{misconduct} (discussed
later).

\begin{verbatim}
      var Ledgers = map[NodeId]Ledger
      type Ledger struct {
        parent     Checksum
        owner      NodeId
        partner    NodeId
        bytes_sent int
        bytes_recv int
      }
\end{verbatim}

Nodes are free to keep the ledger history, though it is not necessary for
correct operation. Only the current ledger entries are useful.

\subsubsection{Protocol Specification}



\subsection{Object Model}

Files are represented as a collection of inter-related objects, like in the
version control system Git. Each object is addressed by the cryptographic hash of its contents (unless otherwise specified, \textit{checksum} will henceforth refer to this cryptographic file content hash). The file objects are:

\begin{enumerate}
  \item \texttt{chunk}: a variable-size block of data.
  \item \texttt{list}: a collection of chunks or other lists.
  \item \texttt{tree}: a collection of chunks, lists, or other trees.
\end{enumerate}

\subsubsection{Block Object}

The \texttt{Block} object contains an addressable unit of data, and
represents a file.
GFS Blocks are like Git blobs or filesystem data blocks. They store the
users' data. (The name \textit{block} is preferred over \textit{blob}, as the
Git-inspired view of a \textit{blob} as a \textit{file} breaks down in GFS.
GFS files can be represented by both \texttt{lists} and \texttt{blocks}.)
Format:
\begin{verbatim}
block <size>
<block data bytes>
...
\end{verbatim}


\subsubsection{List Object}

The \texttt{List} object represents a (large) file made up of several
GFS \texttt{Blocks} concatenated together. \texttt{Lists} contain
an ordered sequence of \texttt{block} or \texttt{list} objects.
In a sense, the GFS \texttt{List} functions like a filesystem file with
indirect blocks. Since \texttt{lists} can contain other \texttt{lists}, topologies including linked lists and balanced trees are possible. Directed graphs where the same node appears in multiple places allow in-file deduplication. Cycles are not possible (enforced by hash addessing).
Format:
\begin{verbatim}
blob <num objects> <size>
<list or block> <checksum> <size>
<list or block> <checksum> <size>
...
\end{verbatim}


\subsubsection{Tree Object}

The \texttt{tree} object in GFS is similar to Git trees: it represents a
directory, a list of checksums and names. The checksums reference \texttt{blob}
or other \texttt{tree} objects. Note that traditional path naming
is implemented entirely by the \texttt{tree} objects. \texttt{Blocks} and
\texttt{lists} are only addressed by their \texttt{checksums}.
% Unlike in Git, GFS trees include file-system metadata such as file
%permissions.
Format:
\begin{verbatim}
tree <num objects> <size>
<tree or list or block> <checksum> <size> <name>
<tree or list or block> <checksum> <size> <name>
...
\end{verbatim}

\subsubsection{Commit Object}

The \texttt{commit} object in GFS is similar to Git's. It represents a
snapshot in the version history of a \texttt{tree}.

\begin{verbatim}
commit <size>
parent <commit checksum>
tree <tree checksum>
author Full Name <email@address.com> <ISO UTC date>
committer Full Name <email@address.com> <ISO UTC date>
<commit message>
\end{verbatim}

\subsubsection{Version control}

\subsubsection{Signed Objects}

All objects can be signed. Add signature to bottom of object.
(yes, this changes the hash, as it should)

\subsubsection{Merkle Trees}

The object model in GFS forms a \textit{Merkle Tree}, where every object
contains hashes of its children. This provides GFS with the useful properties
of merkle trees:

\begin{enumerate}
  \item Tamper resistance
\end{enumerate}

\subsubsection{Published Branches}

Users can publish branches (filesystems) with:
publickey -> signed tree of branches


\subsection{Object Distribution}

\subsubsection{Spreading Objects}

DHash spread along the DHT nodes?
Mainline DHT peer registry?

\subsubsection{Pinning Objects}


\section{Conclusions}




%\section{Acknowledgments}


%\bibliographystyle{abbrv}
%\bibliography{gfs}
%\balancecolumns
%\subsection{References}
\end{document}